The invention relates to a measuring device of the type having a transducer, and especially a measuring device of the type having a high-resistance transducer which is provided with electrical leads.
Measuring devices of the type outlined above are used for polarographic measurements such as the determination of the partial pressures of oxygen and hydrogen in fluids. Such measuring devices are also used for potentiometric measurements such as the determination of pH values as well as for the measurement of ionic activities and thermal values such as heat flow. Measuring devices of the type outlined above are further used for temperature and pressure measurements.
Such devices may be constructed as macroprobes or microprobes. When using a measuring device of the above type, and particularly in those cases where the measuring device is constructed as a microprobe, it is frequently necessary to sharply delimitate the field of measurement. In such an event, the measuring device must have a small, well-defined working surface.
Measuring devices of the type outlined above are of particular utility for measurements in biological systems, e.g. for measurements involving blood, skin tissue and even individual cells. Stringent requirements are imposed on the mechanical stability of the measuring devices, especially for the last-mentioned application, since otherwise the required manipulations would not be possible in the microregion. However, although stringent requirements are imposed on the mechanical stability of the measuring devices, even more stringent requirements are imposed on the electrical characteristics of the measuring devices and the constancy of these characteristics.
In order to satisfy these requirements, attempts have been made to make the transducer, its leads and its insulation in the form of thin films or layers by means of vapor phase deposition, rolling, electrolytic deposition and direct current sputtering (Saito, J. Appl. Phys., 1967/979; Bicher, J. Appl. Phys., 1970/387; Naturwissenschaften, 1974/12, page 660; German Auslegeschrift 1,598,988).
These known films have certain disadvantages, however. Thus, due to the porosity of the insulating and metallic layers, the insulating properties undergo chemical- or physicochemical-induced changes since, for example, foreign matter diffuses into the layers or hydration takes place at the insulating layer. The prior art methods do not result in the requisite pore-free layers. Consequently, the electrical characteristics either cannot be achieved or undergo change during use.
Of the changes in electrical characteristics, those which occur in the electrical resistivity of the insulating layer are of particular significance. This is especially important where high-resistance transducers, that is, transducers having an internal resistance of 1 Megohm and more, are concerned, inasmuch as the variations in resistivity become fully effective here.
The changes in resistivity affect the measurement signal. However, it is not only the measurement signal which is adversely affected by the changes in resistivity. Thus, the changed resistivity also alters the field of measurement because additional portions of the electrode and lead surfaces, namely, those located beneath the insulation layer, can now interact with the object or substance which is to be subjected to a measurement. Accordingly, the field of measurement is less sharply defined than it was prior to the decrease in resistivity.